Triglyceride oils are predominantly liquid at room temperature, and are distinguished by their physical state from fats, which are solid or semi-solid at room temperature. The liquid character of most triglyceride oils is a consequence of their content of lipids comprising fatty acids with one or more double bonds. Double bonds are sensitive to reactions, such as oxidation. Thus, lipids, such as oils, fats, fatty acids, partial glycerides, esters, phospholipids, and other compounds that contain double bonds are susceptible to oxidation reactions.
Lipid oxidation is a reaction that may occur between unsaturated lipids and oxygen, and is accelerated by several factors (light, heat, metals, and other initiating compounds). The consequence of lipid oxidation is often the generation of undesired reaction products. Many reaction products give rise to undesirable odors or flavors in edible oils and fats, as well as products made therefrom. Because of the complexity of natural oils and the large number of possible reaction pathways for a given oxidation reaction, oxidation reactions are incompletely understood. However, some are known to proceed in a radical chain reaction cascade fashion comprising several steps. Natural oils differ in their composition, and thus in their oxidation pathways. Antioxidants are commonly added to lipids to delay the onset of oxidative deterioration. In addition, oils can be stored under an inert gas, such as nitrogen or a noble gas, to minimize oxidation. Sometimes, inert gases are bubbled through oils to displace the small amounts of oxygen in the oil. Oxidative deterioration of oils which undergo storage is a common phenomenon, and limits the useful lifetime of the oil. In addition, oil obtained from oilseeds which have been stored for a substantial period of time after harvest is often higher in oxidation products that oil from seeds obtained from freshly harvested oil.
In the first step of lipid oxidation, double bonds react with oxygen to form allylic hydroperoxides (also known as peroxides). Because they originate from a first step of oxidation, hydroperoxides are considered to be primary oxidation products. They are routinely quantified by a standardized peroxide value test. Good quality oil, which is relatively bland in flavor and low in odor, will generally have a low Peroxide Value (PV). The PV of food oils delivered to food processors is often requested to fall below a specified value to ensure that the foodstuffs produced will be of high quality.
Peroxides are unstable and readily undergo further reactions. A low PV is not the only marker for good oil quality, because the PV of an oil may reach a high level and then decline as peroxides are further broken down into so-called secondary oxidation products. The breakdown of peroxides is complex and incompletely understood, the number of possible secondary oxidation compounds is large, complex, and incompletely classified, and the analysis of secondary oxidation products can be difficult.
One classification system sorts the secondary oxidation products into three groups on the basis of the size of the resulting molecules. Although many high molecular weight unsaturated lipids have no distinctive flavor themselves, their breakdown compounds have intense flavors, which affect the quality and stability of oils. Some secondary oxidation products are of lower molecular weight than the original lipid, and thus are more volatile than the starting lipid and peroxides. These secondary oxidation products are problematic in the edible oil industry. The volatile low molecular weight compounds include aldehydes, carbonyls, ketones, alcohols, acids, esters, ethers, hydrocarbons, and lactones. (Reactions associated with double bonds, in Fatty Acid and Lipid Chemistry, F. Gunstone, Aspen Publishers, Gaithersburg, Md. USA, 1999). Many of these compounds can be tasted or smelled even at very low concentrations and have potent, often undesirable odors or flavors which detract from the quality of edible oil or food made therefrom. Other secondary oxidation products have approximately the same molecular weight as the original allylic hydroperoxide; in this classification, these can be termed rearrangement products. The third group of secondary oxidation products comprises compounds of higher molecular weight than the starting lipid and peroxides, such as polymers which may be formed by condensation, crosslinking, or other types of polymerization reactions.
Another classification system sorts the secondary oxidation products into at least four categories based on standardized analytical methods (Lipid Oxidation, E. Frankel, The Oily Press, Dundee, Scotland 1998). One standard test, the Total Carbonyl Content, measures aldehydes and ketones resulting from the decomposition of hydroperoxides. A second test is the Malondialdehyde test, which is selective for malondialdehyde, a doubly oxidized product resulting from oxidation of polyunsaturated oils. A third test detects Thiobarbituric Acid Reactive Substances (TBARS). This test, also known as the TBAR test, measures a large number of volatile secondary oxidation products. As most edible oil is treated by steam deodorization before being sold, many TBARS may be removed in processing. A fourth test is the Anisidine Value Test, which measures carbonyls formed from lipid oxidation, such as 2-alkenals and 2, 4 dienals. This test is especially useful for quantifying deterioration of oils high in linolenic acid, an 18 carbon polyunsaturated fatty acid containing three double bonds. In addition, oxidative dimers of triglycerides, aldehyde-glycerides, and core aldehydes are known to contribute to high anisidine values.
The anisidine value test is based on the detection of carbonyl compounds, primarily aldehydes, which arise through oil oxidation. These compounds and their breakdown products affect the stability and quality of oils. Standard methods published by the American Oil Chemists' Society (AOCS) are often used to measure characteristics of lipids. AOCS method Cd 18-90 (“Official Methods and Recommended Practices of the AOCS”, Fifth Edition, Second Printing (2004) American Oil Chemists' Society, Champaign, Ill.,) is widely used to measure anisidine values of lipids. Color reversion is a phenomenon in which refined oils become darker upon storage; it is known to be related to the degree of secondary oxidation of oils, which can be measured by anisidine value. Some of the anisidine-reactive materials, such as 2,4-dienals and trans-2-alkenals have been reported to have cytotoxic effects on experimental animals. The anisidine value (AV) of freshly processed oils can be used as a rough predictor of the future storage stability of that oil. Generally the anisidine value of good oil is less than 10 (Classical analysis of oils and fats, in, Analysis of oils and fats, edited by R. J. Hamilton and J. B. Rossell, Elsevier Applied Science Publishers, London, 1986). Anisidine value is also known as para-Anisidine Value (pAV). For these reasons, the AV is an important quality factor for edible oils.
Adsorptive treatment that reduces the AV of fats and oils would be useful in a broad arena of fats and oils. Indeed, the removal of secondary oxidation products is not deemed to be cost effective on a large scale: “While hydroperoxides are easily reduced by conventional purification, it is not economical to remove the secondary lipid oxidation products on an industrial scale” (p. 593 of, “The effect of lipid oxidation on the activity of interesterification of triglyceride by immobilized lipase” Nezu Toru; Kobori, Satoru; Mastumoto, Wataru. Editor(s): Yano, Toshimasa; Matsuno, Ryuichi; Nakamura, Kozo. Dev. Food Eng., Proc. Int. Congr. Eng. Food, 6th (1994), Meeting Date 1993, 591-3. Publisher: Blackie, Glasgow.
Lipid oxidation often takes place in lipids upon storage. Antioxidants may delay the onset of lipid oxidation for a period of time. In addition, lipid oxidation may occur in oilseeds on storage. Thus oil extracted from seeds which have been stored for more than three months after harvest season is often higher in oxidation products than oil extracted from seeds obtained early in a harvest season. As the seed ages in storage, oxidative indicators, such as anisidine value, may rise. Oil made from seed which has been stored for greater than three months may have deteriorated, and have unacceptably high levels of oxidation products. Storage damage reduces the quality of oil (Altschul, A, “Biological processes of the cottonseed” in Cottonseed, Alton E. Bailey, Editor, Interscience publishers, New York 1948 p 157-212). For example, cottonseed is harvested in the USA in Autumn and goes into storage in October and November. Oil produced from this seed in the first three months of storage will usually have a predictable low anisidine value. However, when the warm summer months (June-August) arrive, the anisidine value of oil produced from the seed rises, peaking in mid-September. The anisidine value of oil produced from stored seeds can be greater than 10, which is unacceptably high for some applications and food manufacturers may have difficulty making food which meets quality specifications when only such cottonseed oil is available. There is a need for a treatment which allows cottonseed oil of sufficiently low anisidine value and sufficiently high Oxidative Stability Index (OSI) value to be produced from stored seed. Other oilseeds, including corn and soybeans, are subject to storage. This and other conditions may result in oil having high peroxide values, high anisidine values and/or poor OSI values. A process for reducing the peroxide value and/or anisidine value of oils would be of value to the food industry. Similarly, a process whereby OSI values are increased would be valuable. Oxidative Stability Index is measured according to AOCS method Cd 12b-92.
Lipid oxidation is also a known problem in commercial frying operations since the frying process subjects oil to high temperatures and moisture, which accelerate lipid oxidation. Consequently, the operational lifetime of frying oil may be limited as oxidation reactions proceed, diminishing the quality of the oil and food prepared in it and necessitating cleaning and/or replacement of the oil.
The present teaching addresses these problems and others, and provides further advantages that one of ordinary skill in the art will readily discern from the detailed description that follows.